Generally, there has been known, for example, an electrostatic coating apparatus which is constructed of an atomizer composed of an air motor and a rotary atomizing head, a housing member formed of an electrically insulating material and arranged to hold the air motor of the atomizer therein, a tubular cover member arranged to cover the outer surface of the housing member, and a high voltage generator provided with an external electrode to charge paint particles, which are sprayed forward by the rotary atomizing head of the atomizer, with a negative high voltage (e.g., see Japanese Patent Laid-Open No. 2001-113207).
In the case of the electrostatic coating apparatus provided by the prior arts, an electrostatic field is formed by lines of electric force between an external electrode, to which a negative high voltage is applied, and a rotary atomizing head which is held at the earth potential, and at the same time between the external electrode and a work piece. Besides, an ionization zone is formed in the vicinity of a nose end of the external electrode assembly.
If, in this state, the rotary atomizing head is put in high speed rotation to spray paint, paint particles sprayed from the rotary atomizing head are imparted with an electrostatic charge by a negative high voltage while passing through the ionization zone to become charged paint particles. Therefore, charged paint particles are urged to fly toward and deposit on surfaces of a work piece which is connected to the earth.
In this regard, in the case of the electrostatic coating apparatus described in Japanese Patent Laid-Open No. 2001-113207, outer surfaces of the cover member are electrified with the discharged electrostatic charge of negative polarity. Therefore, repulsions occur between the charged paint particles and the cover member of the same negative polarity, preventing paint particles from depositing on outer surfaces of the cover member. Besides, the cover member is formed of an electrically insulating material to prevent the high voltage electrostatic charge on outer surfaces of the cover member from leaking to the side of the earth potential.
However, in an actual electrostatic coating operation, paint particles start to gradually deposit on outer surfaces of the cover member as the operation is continued, accumulating as paint deposits to degrade the insulation performance of the cover member. Deposition of paint particles progresses abruptly in step with degradations in insulation quality of the cover member. Therefore, in the prior art, it is often the case that a coating operating has to be stopped frequently in order to remove paint deposits.
Further, in the case of the electrostatic coating apparatus of Japanese Patent Laid-Open No. 2001-113207 mentioned above, a water repellent paint is coated on outer surfaces of the cover member to prevent deposition of paint particles. However, a coating apparatus of this sort has a problem in that the thickness of the water repellent paint coating becomes thinner and thinner when outer surfaces of the apparatus are washed repeatedly at the end of coating operations, necessitating to coat the water repellent paint periodically. In addition, instability in quality of the water repellent paint often results in a lower yield of products and costly coating operations.